The Fizzing Antacid Race
Students test how temperature affects the reaction rate of an antacid tablet in dilute acid. They use whole tablets in 100 mL of acid at 10 °C, 25 °C, and 40 °C and time how long bubbling lasts. Results: 210 s, 95 s, and 48 s. A teammate points out that the 40 °C trial used a tablet broken into four pieces because the whole tablet stuck in the funnel, and that the 10 °C beaker was wider, so the tablet lay flatter and bubbles escaped more freely. The class wants to claim that reaction rate roughly doubles for every 10 °C rise. They have one tablet of each size left and 30 minutes.
1. Identify the confounding variables and explain the direction of each bias on the temperature conclusion.
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Two confounds exist. First, the 40 °C trial used a broken tablet, increasing surface area and speeding the reaction. That makes the 40 °C time shorter than it should be, exaggerating the apparent temperature effect.
Second, the wider 10 °C beaker let gas escape more freely and spread the tablet flatter. Freer escape could end visible bubbling sooner, shortening the measured time and making 10 °C look faster than it is, which would understate the temperature effect. The two biases push in opposite directions, so the net distortion is unclear, but the comparison is not a clean test of temperature.
2. Given limited materials and time, propose a controlled version and specify what must be held constant.
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Use identical beakers, whole tablets of the same mass, and the same acid volume and concentration. Control temperature with a water bath and measure it with a thermometer before adding the tablet. Hold constant: tablet mass and shape, acid volume and concentration, beaker size, and the start/stop criterion for bubbling (e.g., until bubbles stop crossing a marked line, judged by two observers).
With limited tablets, run one trial per temperature but use the same beaker and procedure throughout, and record temperature continuously. Ideally repeat, but if not possible, state that single trials limit confidence.
3. The class claims the rate doubles per 10 °C. Assess this claim against the data and the design.
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The raw times (210, 95, 48 s) are roughly halving per 10 °C, which superficially supports the claim. But rate is inversely related to time, and the design has confounds, so the numbers cannot be taken at face value. The broken 40 °C tablet would inflate the apparent speed-up; the wide 10 °C beaker could deflate the low-temperature time.
A doubling rule is also a rule of thumb, not a law, and three single trials give no error estimate. The honest conclusion: the data are consistent with a strong temperature effect, but the claim of exact doubling is not established by this design.